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Batteries - round versus flat

Tesla currently uses a round battery which is apparently some quasi-standard, but has now redesigned that battery from the ground up for the new Gigafactory resulting in slightly different dimensions, but still round.

Obviously in terms of battery density per volume, a flat battery is more efficient than a bunch of round batteries. Cell phones and computers all went to flat batteries long ago. Not just flat packages which contain round batteries as was the case early on, but actually flat batteries.

Does anyone know why Tesla's redesign from scratch of the Li-On battery resulted in another round design?

Cell phone batteries are actually called a pouch type, it's without a fixed shape, but then contained in a hard external shell to keep its shape. I don't know how thick the shell need to be so it can cause inefficient space utilization, and maybe also cooling difficulties.

Cylindrical cells are made like a "swiss roll", so probably one of the easier shape to manufacture, and structurally probably also stronger than a flat brick. I wonder if also makes cooling easier. If you have a flat brick, I imagine you will be layering multiple layers of batteries on top of each other, and the layers in the middle may not have good access to cooling.

The trend is towards pouch cells. Tesla uses cylindrical cells as they were the cheapest option and lots of manufacturing capacity available back 10 years ago. Any mobile device and laptops use pouch cells these days. It's funny when articles still refer to cylindrical cells as 'laptop batteries'. Not any more. Pouch cells are very flexible in terms of shape, thus cooling can be tailored to what works best.
But as JB said once, I wonder why people are so concerned about the cell format, when really what matters is what's inside.

Best thing Tesla ever did was break the viscious cycle of electric cars. No one was making proper EV batteries with scale and got them cheaper because there was no demand, and no one was making cars because batteries were expensive and rare. So Tesla took a different approach and decided to work with already abundant, laptop batteries at the time, of 18650 cylindirical format. (18mm radius, 65mm height)

They designed their pack with this format. Usually batteries designed as "EV Batteries" are more prone to rocking, temperature changes, high discharge currents etc. They're designer cells. However 18650 is not like that. So Tesla designed an amazing pack that micro managed the 6000 or so Roadster cells.

Later on with Model S they got fuse and other safety controls out of the cell to the pack level, so they could squeeze more energy into the cells. After that with later iterations of Model S they changed some of the graphite on the anode with silicon. Silicon allows more energy density but its expansion and contraction during charge and discharge can harm the cells. Somehow Tesla managed to put miniscule amounts of silicon in.

All in all they started with 2.2Ah Roadster cells and with the new 100kWh pack they'll probably be 3.7Ah. 68% more energy density in 8-9 years.

Now with Model 3 Gigafactory batteries they will have the 21-70 format. So 40% larger in volume. They will probably be 5.2Ah. They'll need half the amount of cells for the same pack, construction costs will go down.

Cylindirical cells' other advantage is easier cooling because they have more surface area. Downside is them being sensitive, requiring micro management and more difficult to construct.

Pouch batteries can be efficiently cooled and compressed as well. In the Volt, Spark, and Bolt EV GM uses pouch cells and places a thin aluminum cooling fin/sheet in full surface contact with one side of each cell so the battery is kept at an even and consistent temperature. The Volt and Spark EV aluminum fins are 2 layer and contain tiny coolant channels inside. The Bolt EV fins are single ply and passive but are attached at the base to a coolant plate. By contrast, the Tesla cylindrical cells are only in contact with the coolant ribbon on one side and there is more space between cells in the pack. On the other hand, Tesla uses a more energy dense cell chemistry but the cathode chemistry is inherently somewhat less stable for fire safety. Other car makers are, I think, using so-called prismatic cells which come with their own individual rigid outer shell similar to the rigid outer shell of a cylindrical cell but in a rectangular flat shape.

It's all about making various tradeoffs and balancing characteristics against each other at the cell and pack levels. Tesla and Panasonic seem to be doing very well with their choices so far.

Part of the genius of placing the batteries under the floor is that Tesla doesn't need to optimize for every last bit of space efficiency. It's OK if the battery is a little bulkier than it needs to be.

"Wasted space" also means more space for fire retardant between the cells. Maximally dense packing is not always what you want.

With both Tesla and SpaceX we see a constant theme of sacrificing theoretical efficiency for desirable practical attributes. If the cells take up more space but are easier to build and you can still fit enough of them into a car, it's a win.

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